Agitation system for alkylbenzene oxidation reactors

ABSTRACT

A process for producing aromatic dicarboxylic acids by the oxidation of dimethylbenzenes wherein dimethylbenzenes are mixed with an oxygen containing gas, solvent and catalyst in a reactor and the reaction mixture is agitated with one or more asymmetric radial impellers in combination with at least one axial impeller.

This application is filed pursuant to 35 USC 371 based uponPCT/US02/13216, filed Apr. 24, 2002, which claims the benefit of U.S.Application No. 60/291,067, filed May 15, 2001.

This invention relates to an improved agitation system for large scalecrude terephthalic acid (“CTA”) oxidation reactors which improvesgas-liquid mass transfer. The system consists of a gas dispersing radialturbine such as a Bakker Turbine (BT6) in combination with one or moreaxial impeller(s), such as pitch blade turbine(s), in the down pumpingmode. Gas is preferably sparged through the nozzles near the blade tip.

Crude terephthalic acid is obtained by oxidation of the methyl groups ofp-xylene. The reaction is performed by sparging air or other oxygencontaining gas through an organic mixture (p-xylene and acetic acid) andcatalyst, typically together with a recycle stream. Heat generated bythe oxidation reaction is removed by the vaporization of solvent andreaction water. The temperature in the reactor is controlled by thevaporization of solvent and reaction water and by the recycle in thereactor of a condensate stream of the overhead vapors. The oxidationreactor is a continuous stirred tank reactor, normally at a temperatureof 180 to 205° C. and a pressure of 15 to 18 bar. CTA is recovered fromthe reactor effluent via crystallization and filtration. It is desiredto improve the quality of the CTA in terms of color and impurities, suchas 4-carboxy-benzaldehyde.

It is known in the art that mass transfer is an important factor for theoverall conversion of p-xylene and CTA quality. It is essential indesigning and optimizing the oxidation reactor for enhanced PTAproductivity and quality to understand gas-liquid mass transfer, whilecomparing different agitator configurations. Furthermore, the masstransfer efficiency is even more critical as the current trend in theindustry is to reduce capital and operating costs which has led tobigger plants (500 kT/yr or more) with a single oxidation reactor, asopposed to the parallel reactors which were used in former times. Thedesire to improve gas-liquid mass transfer in the CTA reactors hasrecently spawned some unique agitation system designs, and the trulyoptimal design will set the competitive advantage in PTA technologylicensing.

BT6 is the product name for the Bakker Turbine designed and marketed byChemineer Inc. It is a radial gas dispersing impeller that is claimed tobe less susceptible to flooding. Like the Rushton and Concave diskimpellers, the BT6 consists of six blades extending radially from adisk. These blades are parabolic in shape, like the SRGT (SCABA) blades,but their upper arc is longer than their bottom arc. It has beendiscovered that this turbine design when used in combination with axialimpellers would offer improved mass transfer in the reaction system usedto produce aromatic dicarboxylic acids such as terephthalic acid. Theuse of radial asymmetric blades such as the BT6 together with one ormore axial impellers would offer superior gas-liquid mass transfer inCTA reactors.

Accordingly the present invention relates to a process for producingaromatic dicarboxylic acids such as terephthalic acid by the oxidationof dimethylbenzenes such as p-xylene wherein the dimethylbenzenes ismixed with an oxygen containing gas, solvent and catalyst in a reactor,the improvement comprising agitating the reaction mixture with one ormore asymmetric radial impellers in combination with at least one axialimpeller.

Suitable asymmetric radial impellers are described in U.S. Pat. No.5,791,780. As disclosed in that reference, in general, the asymmetricradial impeller will include a plurality of generally radially extendingblades. Each of the blades will include upper and lower sheet-likeportions which meet at a vertex, such that the cross-section of theblade will be generally parabolic or u-shaped. The width of the upperportion of each blade will be longer than the width of the lower portionmaking the blade asymmetric. Thus, at the leading edge of the bladethere will be an upper portion overhang which can capture and disperserising bubbles. The impeller can have any number of blades, but it ispreferred that it has from 4 to 12 blades with 6 being most preferred.The upper sheet should extend 15 to 50 percent further than the width ofthe lower sheet, with about 25 percent being most preferred. While it ispossible to use more than one asymmetric radial impeller in the processof the invention, a single impeller is generally preferred.

Axial impellers are generally known in the art and any such impellersmay be used in the present invention. For example, a double heliximpeller such as the one depicted in U.S. Pat. No. 5,108,662, or anairfoil blade impeller such as the one depicted in U.S. Pat. No.4,231,974 could be used in this invention. Other suitable axialimpellers include Pitch Blade Turbine, high efficiency impellers (suchas model A-310 from Lightnin Mixing Co, HE-3 from Chemineer, Inc. andViscoprop from EKATO Rueher and Mischtechnik GmbH), single helixes ormarine props (such as A-315 or A-320 from Lightnin Mixing Co., MT-4, orMY-4 from Chemineer, Inc.). The number of axial impellers used ingeneral depends on the viscosity of the working media. The more viscousthe working media the more axial impellers are warranted. It iscontemplated that the invention may comprise from 1 to several axialimpellers 2, but it is preferred that there be two.

The present invention may also include the use of a draft tube. Drafttubes and their modifications are known in the art, and those teachingsare generally applicable to this invention. For example, the draft tubecan be slotted to provide for return of liquid to the center of thedraft tube if the level of liquid for some reason does not exceed thetop of the draft tube. Also, the use of vertical baffles on the innersurface of the draft tube can be advantageously used to redirecttangential flow to axial flow. If baffles are used in the draft tube itis preferred that they have a width of 0.8 to 0.1 of the draft tubeinner diameter with a clearance of 0.016 to 0.021 of the draft tubeinner diameter. Moreover, the use of a baffle to partially close of thebottom of the cylinder formed by the draft tube is shown, inter alia, inU.S. Pat. No. 5,536,875 and may also be used in the present invention.

Although the dimensions of the draft tube are not critical to thepresent invention, it has been found that the optimum radius of thedraft tube is 0.707 of the tank radius. Using a draft tube of thisradius make the cross sectional area of the tank which is inside thedraft tube equal the cross sectional area of the tank which is outsidethe draft tube. The draft tube can optionally contain a conically flaredportion, at the entrance end of the draft tube. It is believed that thissection will aid in straightening the flow of the reactor contents. Theangle of the bevel should be between 30 and 60 degrees, with 45 degreesbeing most preferred. The beveled edge should not be too long, such thatit restricts flow around the top of the draft tube. It is preferred ifthe length of the beveled edge is from zero to about one fourth of thedraft tube's inner diameter, with about 1/12 of the length being mostpreferred.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the tank geometry and agitation system design for the tankused in the examples.

FIG. 2 shows back mixing k_(L)a [1/s] versus power per mass [W/kg] withthe dissolved oxygen probe in the bottom position at 20° C. (gasvelocity 0.043 m/s at 1.4 VVM).

FIG. 3 shows back mixing k_(L)a [1/s] versus power per mass [W/kg] withthe dissolved oxygen probe in the middle position at 20° C. (gasvelocity 0.043 m/s at 1. VVM).

FIG. 4. shows back mxing k_(L)a [1/s] versus power per mass [W/kg] withthe dissolved oxygen probe in the top position at 20° C. (gas velocity0.043 m/s at 1.4 VVM).

FIG. 5 shows gas hold-up versus power per mass [W/kg] for the BT6+PBTsystem at different air flow rates for an aerated acetic acid solution.

FIG. 6 shows the expansion factor versus power per mass [W/kg] for theBT6+PBT system at different air flow rates for an aerated acetic acidsolution.

FIG. 7 shows the comparison of ungassed power consumption [kW] versusimpeller speed [rpm] for two different agitation systems at 0 VVM.

FIG. 8 shows the comparison of power consumption [kW] versus impellerspeed [rpm] of two agitation systems at 1.4 VVM

FIG. 9 shows the power draw (k-factor versus aeration number) for theBT6+PBT system.

FIG. 10 shows the power draw (k-factor versus aeration number) for theRushton+PBT system.

EXAMPLES

In order to demonstrate the surprising effectiveness of the presentinvention, a series of gas-liquid experiments were carried out to studythe flow and mass transfer performance of the asymmetrical radialimpellers such as the Bakker turbine (BT6) impeller. The experimentalk-factor, gas-hold-up, and the mass transfer coefficient (k_(L)a) valuesof the BT6 agitator were compared with those of Rushton turbine. All thetests were conducted in an air-water or an air-water-acetic acid system.Since the working media is different from that in the CTA reactor, theresults characterize the relative performance of the gas dispersingimpellers in combination with an axial impeller such as the pitchedblade turbine (PBT). Under reaction temperature and pressure conditionsin the CTA reactor, absolute volumetric mass transfer coefficient(k_(L)a) values could be significantly different from those in thepresent experimental air-water system.

There are two general techniques, that is, the transient andsteady-state methods, for evaluation of the combined liquid film masstransfer coefficient and interfacial area (k_(L)a). In this experiment,the steady-state method was used. The concentration of oxygen in theliquid is monitored by a dissolved oxygen (D.O.) probe. When steadystate is reached, that is, the oxygen concentration remains constant,the D.O. value is recorded. The value of k_(L)a is determined from theoxygen transfer rate and the oxygen concentration in the liquid. Due tothe presence of salt and H₂O₂ in water, the k_(L)a values calculatedwith this method are usually higher than those obtained with thetransient method.

All the experiments were carried out in an ASME dish bottom 66″ (1676mm) diameter, 1800 gal (6.814 m³) tank. The tank had four flat bafflesand four side entry ¾″ ID tangential air nozzles. The nozzles arepositioned near the impeller tips. The tank geometry and agitationsystem designs are summarized in FIG. 1 and Table I.

TABLE 1 Summary of the tank geometry and agitation system designs RadialImpeller Data Vessel Volume Data (D = impeller diameter) Bottom head:100.67 gal (0.381 m³) Blade length: L = 0.25 D Vessel total: 1744.62 gal(6.60 m³) Blade width: W = 0.20 D Disk diameter: d = 0.67 D

The dissolved oxygen concentration was measured using a D.O. probe. Theprobe was mounted on a rod and positioned at each of three differentheights, while pointing against the direction of tangential flow. Thethree different heights were 16.5″ above the upper impeller axis, 13″below the upper impeller axis (between the two impellers), and 11″ belowthe bottom impeller (below the air sparger, as well). These probepositions were reproducible due to an interlocking nut affixed to therod.

Hydrogen peroxide solution, at 35 wt. percent., was metered into thetank. Conductivity was measured before and after the tests using aconductivity meter. Temperatures in the head space and the liquid weremeasured. The flow rate of air was controlled by a data acquisition andcontrol system. Agitator rpm and shaft power were also measured.

FIGS. 2 to 4 show the volumetric mass transfer coefficients at differentlocations in the tank, as a function of specific power at 1.4 VVM(superficial gas velocity=0.043 m/s). The agitation systems used inobtaining this data included the PBT and either the Bakker (BT6) orRushton turbine. Table 2 shows the relative mass transfer performance ofthe BT6 turbine as compared to the Rushton turbine.

FIGS. 2 to 4 clearly show that the mass transfer performance of BT6+PBTwas surprisingly superior (˜17%) to Rushton+PBT. The average volumetricmass transfer coefficients in these systems were found to be 0.210 and0.176 sec⁻¹ for the BT6 and Rushton turbines, at 20° C., respectively.It is believed that the asymmetric parabolic profiles of the BT6 bladeshelp restore gas bubbles in the bottom head, thus enhancing the masstransfer coefficient in this region of the tank. Since CTA reactors aretypically quite large, the volume of their bottom semi-elliptical headcan be on the order of ca. 60 m³ in a 500 kT/yr PTA production plant.Therefore, mass transfer improvement in the bottom head region iscritical.

TABLE 2 Mass transfer performance of the BT6 + PBT agitation systems,relative to the Rushton + PBT system, at the various sample positions inthe tank BT6 + PBT Top  +9.23% Middle +27.76% Bottom +12.58% OVERALL+16.52%

Tests were also performed using acetic acid solution to determine thegas hold-up and the expansion factor. The acetic acid solution wastailored to simulate the physical conditions of the CTA reactor workingmedia. FIGS. 5 and 6 show the gas hold-up and expansion factor versusspecific power consumption for different gas flow rates. The resultsshown in these figures were in good agreement with real industrialreactors where the expansion factor is about 2.

To obtain the power draw characteristics of different agitation systems,tests were carried out in unaerated and aerated acetic acid solutionmedia. FIG. 7 shows the power draw for different agitation systems as afunction of impeller speed under ungassed condition.

Observations similar to those from FIG. 7 can be seen from the resultsshown in FIG. 8, which represents gassed conditions at 1.4 VVM. Asexpected, the power draws differ due to differences in the shape ofthese radial impeller blades. The agitation system containing the BT6consistently drew the least power at all conditions considered. In atypical 500 kT/yr PTA plant, the CTA reactor uses a ca. 1000 kW drivesystem and operates at dual speeds to accommodate the power draw duringeither gassed or ungassed conditions. Thus, a clear understanding of thegassed and ungassed power draw is vital to the cost-effective design ofthe CTA reactor.

The ratio of gassed power to ungassed power, known as the k-factor, isused in determining drive system requirements. FIGS. 9 and 10 presentk-factors versus aeration numbers for two agitation systems at variousgassing conditions (0.0 to 1.6 VVM by steps of 0.2) and agitation rates(60 to 140 RPM). The figures' curves include nine aeration numbers atvarious fixed values of impeller Reynolds number (Re).

Each curve in FIGS. 9 and 10 reaches an operating region where increasedaeration number doesn't change the k-factor dramatically. While none ofthe curves show the k-factor suddenly increasing with aeration number,an indication that the impeller system has started to flood, it shouldbe remembered that this data is also influenced by the PBT coupled toeach radial impeller (BT6 or Rushton).

Initiation of gas flow caused k-factors in FIGS. 9 and 10 to decreasemost dramatically (30 to 50%) for the Rushton-based agitator system.More significantly, k-factors for the BT6-based system never droppedbelow 0.6, while those for Rushton-based systems dropped as low as 0.35.

Table 3 contains data from the figures, and represents the highest fixedvalues of impeller Reynolds number (at a 140 RPM agitation rate) andaeration number (at 1.6 VVM). The table data show a significantadvantage in the power requirements of the BT6 (10.5 kW/14 Hp), ascompared to the Rushton impeller system (>16.5 kW/>22 Hp). This effectis due to vortex shedding and pressure drop in the wake region (behindthe impeller blades), which increases in the order: BT6<Rushton.

TABLE 3 Power requirements and k-factors for the BT6 + PBT and Rushton +PBT agitation systems, at fixed operating conditions (acetic acidcoalescing media) BT6 + PBT Rushton + PBT Agitation rate (RPM) 140 140Reynolds number 1.64 × 10⁶ 1.64 × 10⁶ Aeration number 0.10 0.10 k-factor0.6 <0.35 Ungassed Power (kW/Hp) 10.5/14 >16.5/>22

The lower power requirements of the BT6-based system means loweroperating costs, while also adding the ability to operate at a higheragitation rates than with the other impeller systems. Additionally,k-factors with a value nearer to 1.0 make the BT6 better able to operateat the same speed whether operating under gassed or ungassed conditions.This provides another very significant savings, in the initial capitalcost of the impeller drive system, because only a single speed drive isrequired.

These experiments showed that the most effective gas-liquid-solid mixingtechnology for the stirred tank reactors used in CTA oxidation includes:

-   (1) the BT6 impeller in combination with one or more axial    impeller(s), such as pitch blade turbine(s), in the down pumping    mode-   (2) gas sparged through tangential nozzles that are positioned near    the impeller tips, but just below the horizontal centerline of the    blades    Advantages of the BT6 agitation system over either concave disks or    Rushton turbines are superior mass transfer performance, lower    ungassed power consumption, less susceptibility to flooding at high    aeration rates, and better solids suspension. Lower ungassed power    requirements of the BT6 will significantly reduce capital costs of    the agitator drive system, by needing only one speed instead of two.

The reduction in acetic acid consumption is estimated as ca. 10 percentand the increase of yield of p-xylene is estimated as ca. 0.5 to 1percent.

1. In a process for producing aromatic dicarboxylic acids by theoxidation of dimethylbenzenes wherein the dimethylbenzenes are mixedwith air, solvent and catalyst in a reactor, comprising agitating thereaction mixture with one or more asymmetric radial impellers incombination with at least one axial impeller.
 2. The process of claim 1wherein the aromatic dicarboxylic acid is terephthalic acid and themethylbenzene is p-xylene.
 3. The process of claim 1 wherein thearomatic dicarboxylic acid is isophthalic acid and the methylbenzene ism-xylene.
 4. The process of claim 1 wherein the aromatic dicarboxylicacid is ortophthalic acid and the methylbenzene is o-xylene.
 5. Theprocess of claim 1 wherein a draft tube is used to further improve thegas-liquid mass transfer.
 6. The process of claim 1 wherein the radialimpeller comprises multiple parabolic shaped blades extending radiallyfrom a disk with each blade having an upper arc longer than its bottomarc.
 7. The process of claim 1 wherein the axial impeller is a pitchedblade turbine impeller.
 8. The process of claim 1 wherein theoxygen-containing gas is sparged into the reactor through tangentialnozzles that are positioned near the asymmetric radial impeller tips,and slightly below a horizontal centerline of the asymmetric radialimpeller.
 9. A process for producing aromatic dicarboxylic acids byoxidation of dimethylbenzenes comprising: agitating dimethylbenzeneswith air, solvent and catalyst in a reactor with an agitation systemcomprising a combination of at least one asymmetric radial impeller withat least one axial impeller selected from a: pitched blade, airfoilblade, high efficiency, and marine.
 10. A process for producing aromaticdicarboxylic acids by oxidation of dimethylbenzenes comprising:agitating dimethylbenzenes with an oxygen containing gas, solvent andcatalyst in a reactor with an agitation system comprising a combinationof at least one asymmetric radial impeller with at least one pitchedblade impeller.
 11. The process of claim 10 where in the oxygencontaining gas comprises air.